The present invention relates to, for example, a decontamination method and system for soil and the like for decontaminating soil such as fields and water contaminated with radioactive materials reliably and rapidly on site, performing decontamination with precision, and enhancing the efficiency of decontamination. The invention improves the decontaminated soil by adding a soil activator to the decontaminated soil, and then the improved soil is returned to the original field readily to promote the restart of farming. Further, the invention separates the radioactive materials adhering to or deposited on the soil from the soil precisely and then the radioactive materials are concentrated. The invention enables a reduction in the volume of the contaminated soil and achieves a safe treatment of the radioactive materials.
The Great East Japan Earthquake occurred on March 2011 caused an accident of the Fukushima Daiichi Nuclear Power Plant of Tokyo Electric Power Company. The accident has dispersed harmful radioactive materials in a wide area and contaminated cities and towns, fields, mountains and forests, the sea, lakes and marshes, and rivers. The radioactive materials adhered to or deposited on persons, animals, and plants have endangered their lives and caused serious damage, stopping various industrial activities such as farming, forestry, livestock farming, fishery, and the like.
Removal of such radioactive materials from living environments and industrial activity areas is mandatory for recovery and restart of industrial activities. Decontamination of soil in fields is an urgent problem especially for people engaged in farming.
Contamination has widely spread over fields and scattered throughout villages and mountains as well as plains. Decontaminating such wide areas with human power requires a great amount of time and labor, which is inefficient. The advanced age of many people engaged in farming also makes the decontamination work extremely difficult.
In order to deal with such contaminated soil or decontamination treatment, there is a method for decontaminating halide radioactive wastes by dissolving radioactive material into solvent and then separating the radioactive material from the solvent. In this case, halide is dissolved into water, which is a solvent, to precipitate rare earth elements in the solvent, and the precipitate is collected. As a means for separating the non-radioactive material from the solvent, the solvent is evaporated or cooled to precipitate the non-radioactive material (refer, for example, to Patent Literature 1).
However, in the above-mentioned decontamination method, the method in which halide is dissolved in water and the radioactive material is collected shows a low collection rate. Further, another method which includes steps of evaporation and cooling of the solvent requires a heating device and cooling facilities, thereby making the facilities large-scale and expensive.
Still another example of soil decontamination methods includes digging the soil contaminated with harmful chemical materials, putting the soil into a hopper of the heating device, and heating the soil while washing by using nitrogen to desorb and separate the contaminants in the soil (refer, for example, to Patent Document 2).
Such a decontamination method also has problems. The method requires time and labor for moving the contaminated soil to a remote decontamination device and additionally for returning the decontaminated soil to the original position. Further, the contaminated soil needs to be excavated deeply, not only the surface soil. Thus, the method requires an appropriate excavating facility, making the decontamination expensive and large-scale. Additionally, the decontamination device further requires a nitrogen washing machine, a heating device, and a separator, and thus making the decontamination large scale and expensive.
There is still another example of decontamination methods for soil contaminated with radioactive cesium. The contaminated soil is stored in a water supply tank, and carbon dioxide gas of high partial pressure is injected into the tank to supply hydrogen ions. After extracting cesium ions on the surfaces of soil particles into a liquid phase, the solution is shifted to a separation vessel which is open to air and the carbon dioxide gas is released into the air. Then, the pH value in the liquid phase is raised to precipitate and separate therefrom ions such as alkaline earth metals other than cesium, the precipitate being carbonates or hydroxides thereof, and the cesium remaining in the liquid phase is concentrated and separated (refer, for example, to non-patent document 1).
The above described soil decontamination method also has shortcomings. In the above described decontamination method, the liquid phase of supernatant fluid of the water supply tank is sent to the separation vessel so that the supernatant fluid does not contain much cesium, which has a high specific gravity. This leads to a low efficiency of concentration and separation of cesium. Further, cesium accumulates in the lower part of the water supply tank and promotes attachment and deposition onto the soil, and thereby lowering the effect of decontamination. Accordingly, use of decontaminated soil has been difficult and impractical.
Another decontamination method for soil contaminated with radioactive cesium includes adding water to the contaminated soil received in the reaction vessel, placing anode and cathode electrodes in the reaction vessel, applying a voltage to the electrodes to attract radioactive cesium ions onto the cathode, depositing soil and other matter associated therewith onto the anode electrode, and separating and collecting the radioactive cesium from the contaminated soil to significantly reduce the volume of the contaminants (Refer, for example, to non-patent document 2).
However, such decontamination method for soil also has problems. Since the soil is received in the reaction vessel together with other matter, the method requires a high voltage application, which results in poor efficiency. The radioactive cesium ions attracted to the cathode contain foreign materials so that the cesium ions are separated with a low accuracy. Further, since the decontaminated soil also contains other matter, the separation process is time-consuming and the decontaminated soil cannot be used quickly.
The following is a list of background art referred to herein:
    [Patent Literature 1] JP-A-10-213697    [Patent Literature 2] JP-A-5-192648[Non-Patent Literature][Non-Patent Literature 1]    Choji, Tetsuji Takada, Eiji Tafu, Masamoto (Toyama National College of Technology) Hara, Masanori (University of Toyama)    Houshasei-seshiumu Osendojyo wo Tansan-gasu nomide Senjyo, Shufuku suru Anzen Anshin na Kahan-gata Souchi no Kouchiku (Construction of Safe and Portable Apparatus for Cleaning and Repairing Soil Contaminated with Radioactive Cesium by Using Only Carbon Dioxide Gas),    The First Fukushima Conference's Summary    The Society for Remediation of Radioactive Contamination in Environment, 21[Non-Patent Literature 2]    Ueda, Yuko Watanabe, Isao Toida, Hideki Honda, Katsuhisa (Center of Advanced Technology for the Environment, Faculty of Agriculture, Ehime University)    Denki Bunkai wo Riyou shita Houshaseibushitsu Jyosen Gijyutsu no Teian (Proposal for Decontamination Technique of Radioactive Materials Using Electrolysis)    The First Fukushima Conference's Summary
The Society for Remediation of Radioactive Contamination in Environment, 92